HTTP requests are messages sent by the client to initiate an action on the server. Their start-line contain three elements:

1. An HTTP method, a verb (like GET, PUT or POST) or a noun (like HEAD or OPTIONS), that describes the action to be performed. For example, GET indicates that a resource should be fetched or POST means that data is pushed to the server (creating or modifying a resource, or generating a temporary document to send back).
2. The request target, usually a URL, or the absolute path of the protocol, port, and domain are usually characterized by the request context. The format of this request target varies between different HTTP methods. It can be
   • An absolute path, ultimately followed by a '?' and query string. This is the most common form, known as the origin form, and is used with GET, POST, HEAD, and OPTIONS methods.
     • POST / HTTP/1.1
     • GET /background.png HTTP/1.0
     • HEAD /test.html?query=alibaba HTTP/1.1
     • OPTIONS /anypage.html HTTP/1.0
   • A complete URL, known as the absolute form, is mostly used with GET when connected to a proxy. GET https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages HTTP/1.1
   • The authority component of a URL, consisting of the domain name and optionally the port (prefixed by a ':'), is called the authority form. It is only used with CONNECT when setting up an HTTP tunnel. CONNECT developer.mozilla.org:80 HTTP/1.1
   • The asterisk form, a simple asterisk ('*') is used with OPTIONS, representing the server as a whole. OPTIONS * HTTP/1.1
3. The HTTP version, which defines the structure of the remaining message, acting as an indicator of the expected version to use for the response.

HTTP headers from a request follow the same basic structure of an HTTP header: a case-insensitive string followed by a colon (':') and a value whose structure depends upon the header. The whole header, including the value, consist of one single line, which can be quite long.

Many different headers can appear in requests. They can be divided in several groups:

• General headers, like Via, apply to the message as a whole.
• Request headers, like User-Agent or Accept, modify the request by specifying it further (like Accept-Language), by giving context (like Referer), or by conditionally restricting it (like If-None).
• Representation headers like Content-Type that describe the original format of the message data and any encoding applied (only present if the message has a body).

The final part of the request is its body. Not all requests have one: requests fetching resources, like GET, HEAD, DELETE, or OPTIONS, usually don't need one. Some requests send data to the server in order to update it: as often the case with POST requests (containing HTML form data).

Bodies can be broadly divided into two categories:

• Single-resource bodies, consisting of one single file, defined by the two headers: Content-Type and Content-Length.
• Multiple-resource bodies, consisting of a multipart body, each containing a different bit of information. This is typically associated with HTML Forms.

The start line of an HTTP response, called the status line, contains the following information:

1. The protocol version, usually HTTP/1.1.
2. A status code, indicating success or failure of the request. Common status codes are 200, 404, or 302
3. A status text. A brief, purely informational, textual description of the status code to help a human understand the HTTP message.

A typical status line looks like: HTTP/1.1 404 Not Found.

HTTP headers for responses follow the same structure as any other header: a case-insensitive string followed by a colon (':') and a value whose structure depends upon the type of the header. The whole header, including its value, presents as a single line.

Many different headers can appear in responses. These can be divided into several groups:

• General headers, like Via, apply to the whole message.
• Response headers, like Vary and Accept-Ranges, give additional information about the server which doesn't fit in the status line.
• Representation headers like Content-Type that describe the original format of the message data and any encoding applied (only present if the message has a body).

The last part of a response is the body. Not all responses have one: responses with a status code that sufficiently answers the request without the need for corresponding payload (like 201 Created or 204 No Content) usually don't.

Bodies can be broadly divided into three categories:

• Single-resource bodies, consisting of a single file of known length, defined by the two headers: Content-Type and Content-Length.
• Single-resource bodies, consisting of a single file of unknown length, encoded by chunks with Transfer-Encoding set to chunked.
• Multiple-resource bodies, consisting of a multipart body, each containing a different section of information. These are relatively rare.

HTTP/1.x messages have a few drawbacks for performance:

• Headers, unlike bodies, are uncompressed.
• Headers are often very similar from one message to the next one, yet still repeated across connections.
• No multiplexing can be done. Several connections need opening on the same server: and warm TCP connections are more efficient than cold ones.

HTTP/2 introduces an extra step: it divides HTTP/1.x messages into frames which are embedded in a stream. Data and header frames are separated, which allows header compression. Several streams can be combined together, a process called multiplexing, allowing more efficient use of underlying TCP connections.

HTTP frames are now transparent to Web developers. This is an additional step in HTTP/2, between HTTP/1.1 messages and the underlying transport protocol. No changes are needed in the APIs used by Web developers to utilize HTTP frames; when available in both the browser and the server, HTTP/2 is switched on and used.

HTTP messages are the key in using HTTP; their structure is simple, and they are highly extensible. The HTTP/2 framing mechanism adds a new intermediate layer between the HTTP/1.x syntax and the underlying transport protocol, without fundamentally modifying it: building upon proven mechanisms.